Although the present disclosure may be useful in connection with various kinds of drilling processes, in particular for drilling holes into or through a great variety of workpieces, the present disclosure and the underlying problem will be explained in more detail hereinafter with respect to the drilling of through-holes through workpieces comprising one or more comparatively thin layers of material, in particular in the field of aircraft or spacecraft manufacturing.
For example, during an assembly procedure in aircraft production, sometimes, a workpiece or arrangement made up of two layers of material, one of which comprises a pre-drilled pilot hole, has to be drilled through, so that a through-hole of desired diameter is formed through both layers, preferably as exactly as possible at the position of the pilot hole.
An existing method for drilling exists according to which a semi-automatic drilling unit is used to perform the drilling step. Before initiating the drilling step, the semi-automatic drilling unit has to be positioned and aligned by the worker with respect to the workpiece or arrangement, e.g. a workpiece or arrangement of workpieces comprising a stack of two layers of material.
However, positioning the semi-automatic drilling unit (ADU) is rather difficult, and it is therefore time-consuming to precisely position a drill bit of the ADU with respect to the pilot hole in order to produce a drilled hole that meets the requirements concerning the precision of positioning and the hole quality.
Drilling precise holes of high quality is particularly fiddly, using the existing method noted above, if the workpiece comprises one or more rather thin layers, and in particularly if the layer provided with the pre-drilled pilot holes is very thin.
In such a case, because of the small thickness of a layer of material having the pilot hole, guidance of a drill point of the drill bit at the beginning of the drilling operation is limited. Therefore, care must be taken in order to precisely position the drilling unit relative to the workpiece in order to produce holes through the layer and through a second layer having the right hole dimensions and sufficient hole quality and ensure that the holes are positioned and aligned as required. This is time-consuming.
Moreover, drilling such holes is particularly demanding if the layers to be drilled through comprise one or more layers made of fiber-reinforced composite materials, such as hybrid stacks including layers of fiber-reinforced composite material and layers of metal. However, oversized holes, double holes, delaminations and/or fraying of the hole edges are to be avoided.